Production of metals



Nov. 27, 1962 J. z. CERYCH ETAL 3,065,958

PRODUCTION OF METALS Filed Jan. 15, 1959 RATIO MOTOR MECH.VACUUM PUMP l2 POWER SUPPLY 20 VACUUM' PUMP |;O\TARY SEAL FLOW METER 1 l NON-OXlDIZ F A MEDILM I LIFT United States Patent Gfifice 3,065,958 Patented Nov. 27, 1962 3,065,958 PRODUCTION OF METALS John Z. Cerych, Methuen, Philip J. Clough, Reading,

and Robert W. Steeves, Nahant, Masafassignors to National Research Corporation, Cambridge, Mass., a

corporation of Massachusetts Filed Jan. 15, 1959, Ser. No. 787,055 6 Claims. (Cl. 266-15) This invention relates to the production of metals and more particularly to the production of extremely fine metal powders.

A principal object of the present invention is to provide an economical, simple, vacuum evaporation process for the production of metal powders.

Another object of the invention is to provide a process of the above type for the production of high purity metal powders having a particle size of less than about 0.1

micron.

Other objects of the invention will in part be obvious and will in part appear appear hereinafter.

The invention accordingly comprises the product possessing the features, properties, and the relation of components and the process involving the several steps and the relation and the order of one or more of sucn steps with respect to each of the others which are exemplified in the following detailed disclosure and the scope of the application of which will be indicated in the claims.

for a fuller understanding of the nature and objects of the invention, reference should be had to the following detailed description taken in connection with the accompanying drawing which is diagrammatic, schematic view of one embodiment of the invention.

Some of the many known ways of producing metal powders comprise (a) the hydrogen reduction of certain readily reducible metal oxides, carbonates, nitrates or formates, ([2) the alkali metal reduction of certain metal halides, metal amalgamation and subsequent removal of mercury by, distillation, (d) metal hydride decomposition, (e) carbonyl decomposition, (f) halide decomposition (g) electrolysis and (h) are disintegration. By the present invention, it is possible to conduct a simple, economical, commercially feasible process not heretofore achieved for the production of high purity metal powders having a particle size as small as or smaller than that produced by any one of the above processes.

The numerous uses of fine metal powders are well known. For example, fine metal powders have been used as catalysts, in pigment and powder metallurgical applications and where they possess pyrophoric properties as fuels in explosives, missiles and the like.

For example, the relatively low specific impulse of the solid propellants has been one of the big reasons why liquid units are used, even though their reliability is so far inferior to solid propellants. The fine meLal powders could be used for the control of the entire propellant reaction. This is readily seen in the light of the following considerations. The first step in the thermal ignition process is the adsorption of heat by the particle. Nusselts calculation (Varfahrens Technique Beik. A. Ver. deut. Inq. 68 l24l28, 1924) indicate that the initial rate of heating of the particle is inversely proportional to the particle size and is independent of the thermal properties of the gas. In other words, for very short time lags, Nusselts equation predicts that smaller particles will reach a given temperature at a lower combustion wall temperature than larger particles, thus the wall temperature necessary for ignition is a function of particle size.

In composite propellants where a solid fuel bed is used,

an even distribution of the particles through the fuel bed 'is necessary. With very fine metal powders, if used in formulations, a perfect distribution and more propellant weight per cubic foot could be obtained. Also, since the fine metal powders of this invention have a surface area of approximately 750,0U0 square centimeters per gram, they can adsorb large amounts of liquid fuels. This large adsorption factor of such fine metal powders in addition to their thixotropic nature (to convert the liquid to a gel) are particularly suitable for other propellant applications.

Another application of the extremely fine metal powders of this invention is in the powder metallurgical field. One possibility is a direct route to new extract-composition alloy fabrication. At present, some metal combinations only form alloys in certain fixed proportions. The methods now employed involve melting the metals together to form an alloy, casting and cooling the alloy, cutting and grinding into a powder, preforming under pressure and finally sintering. Now, with the extremely fine metal powders, the fabricator should be able to simply mix tne powders of the desired metals together in "whatever proportions desired and then simply preform and sinter.

Uitrafine metal powders of nickel and iron will have extensive possibilities in magnetic and electromagnetic applications.

in the present process, the pyrophoric metal powders produced are of a very high purity having a uniform partic.e size of less than 0.1 micron. Since these ultrafine metal powders are substantially oxygen-free and have such tremendous surface areas, they possess unexcelled burning characteristics which make them especially useful as rocket and missile fuels. Additionally, because the pure metals produced according to the present process are uniformly smaller than the wavelength of the visible spectrum, they are jet balck and thus perfect heat adsorbers. Therefore, they find additional usefulness where excellent heat adsorption is required or desired.

The present process comprises the thermal evaporation of a metal selected from the group consisting of alumi nurn, manganese, silver, chromium, beryllium, copper, boron, silicon, iron, nickel, zinc, magnesium, titanium, zirconium, thorium and bismuth at a pressure below about 500 microns and the condensation in free space, under similar pressure conditions, of the metal vapors produced. This condensation, in the absence of oxidizing conditions, gives a resultant product consisting of oxide-free, black, spherical, metal powders having an extremely high purity and a particle size distribution such that substantially all of the particles have a diameter of less than about 0.1 micron. The metal powders are then collected in a nonoxidizing or inert medium. In one embodiment of the invention, the inert medium comprises an organic material such as hexane, heptane, parafiin and the like which Will form an oxidation-inhibiting environment for the metal powders. In another embodiment, the metal powders are collected in an inert-gas-filled container which is then sealed against leakage of oxygen or nitrogen. For metal powders to be used as catalysts, a gas such as hydrogen might be adsorbed thereon before sealing.

The invention may be more fully understood when considered in the light of the following description.

In carrying out the process, a charge of the metal to be evaporated is placed in a crucible which is heated by an induction coil suitably insulated therefrom which ele ments together are secured in a vertical cylindrical vacuum chamber such that a large free unobstructed space exists above the source of metal vapors. The vacuum chamber is made sufi'iciently large to prevent direct impingement of the vapors on the walls of the chamber, thus preventing scale formation. The metal is then vaporized directly in the chamber which has a residual pressure of non-condensible gases of between 50 and 150 microns Hg abs. The metal vapor is then condensed in free space and the fine powder resulting therefrom collects on the walls of the chamber. The external cooling coils will allow fast cooling of the walls after the evaporation and condensation is completed. The fine metal powder is then removed from the walls of the chamber by the rotating brush assembly which is driven through a vacuum seal by means such as a ratio motor. The metal powder so removed will then fall through a vacuum seal into a collection vessel. The collection vessel is preferably a conical blender and is constructed with suitable valves so that it can be removed from the vaporization chamber without admitting any air. Thus additional powder can be produced while a previous batch is being further handled in the blender. The blender is also preferably designed so that an inert solvent or gas can be introduced and sufiiciently mixed to prevent oxidation of the pyrophoric powder. The blender can also be used for the controlled oxidation of the pyrophoric metal powders. Cooling and rotating the blender during oxidation will result in good mixing of the metal powder and prevent localized overheating and subsequent selfignition.

The metal to be converted into powder can be evaporated by suitable heating means such as induction heaters, resistance heaters, electron bombardment and the like. One preferred method is to evaporate the metal from a suitably heated source having a large effective metal evaporation area and containing therein a molten pool of metal. Evaporation of the metal in this way avoids excessive splattering and favors the formation of more uniform size particles. The achievement of uniformity of such small particle sizes has heretofore not been possible.

The temperatures required for evaporating the metals, of course, depends upon the vapor pressure of the particular metal and the operating pressures employed. The temperature at which the evaporation is carried out, determines the rate of metal vapors efilux from the source containing the molten metal. Temperatures at which the vapor pressure of the metal is below approximately 0.] millimeter of mercury will yield low evaporation rates while higher temperatures and correspondingly higher metal vapor pressure will yield higher evaporation rates. The rates of evaporation which can be meployed are quite broad and can be varied considerably.

The pressures employed in the chamber for the metal evaporation and condensation of the metal vapor are below about 500 microns and preferably between about 1 and 200 microns. The pressures employed can be obtained by evacuating the system to an extremely low pressure and then adjusting to a high pressure with an inert medium such as argon, helium and the like. The height of the vapor stream emanating from the metal vapor source is largely dependent on the pressure employed. At the low pressures the vapor stream is long. As the operating pressure rises the length of the vapor stream decreases. Thus Where the metal is evaporated from a source, condensed in free space and collected directly on the walls of the chamber, the chamber must be large enough to prevent direct impingement of the vapors on the chamber walls. By this is meant that the particles must be condensed so that they can be collected as spherical particles rather than as platelets.

One preferred type of equipment for producing the metal powders in accordance with the invention is shown in the drawing wherein represents a vertical cylindrical vacuum-tight tank or chamber which is evacuated through conduit 12 by means of a suitable pumping system. The lower portion of chamber 10 is preferably funnel shaped to facilitate the collection and discharge of the metal powders from the chamber. Within tank 10 there is a vapor source 14 here shown as a crucible means for holding a charge of the metal to be evaporated. The vapor source 14, is suitably heated by means 16 illustrated as an induction heating means. Obviously,

more than one vapor source and other types of vapor sources and heating means than those shown can be employed.

Valve means 18 and 20 are provided between the bottom of tank 16 and the powder collecting chamber 22. A non-oxidizing medium is provided within chamber 22. This is preferably accomplished by evacuating chamber 22 through conduit 24 to a low pressure by means of a suitable vacuum pumping system.

External cooling means 26 are provided on tank 10 to allow fast cooling of the metal powder collected on the walls of tank 10. A rotating brush assembly 28 driven through a vacuum seal will remove the metal powder from the walls of tank 10. The metal powder so removed will fall through the vacuum seal of valves 18 and 20 into the collection chamber 22.

If continuous operations are desired, the collection chamber 22 can be removed from tank 10 without admitting air by closing valves 18 and 20 and then disconnecting and lowering. Thus, an additional metal charge can be added to tank 10 while the previous batch of metal powder is being further handled in collection chamber 22. The design of chamber 22 is such that heptane or any other inert solvent or gas can be introduced through conduit 24. A source of inert gas or solvent can be provided for filling chamber 22 with any desired amount of such inert gas or solvent. Suitable means can be provided for raising and dropping chamber 22 when disconnected from tank 10.

As pointed out in the copending application of Cerych and Clough, Serial No. 788,260, filed on January 22, 1959 the fine metal powders produced in accordance with the present invention can be reacted with another substance such as hydrogen, a halogen or an organic compound to produce finely divided compounds of the metals. The apparatus of the present invention permits the extremely small metal particles to be produced with no adsorbed layer of gas. Thus, such powders have increased free energy (as explained in the above mentioned Cerych and Clough application) for formation of compounds which otherwise are difficult to manufacture.

Since certain changes can be made in the above without departing from the scope of the invention herein involved, it is intended that all matter contained in the above description or shown in the accompanying drawing shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. Apparatus for the production of fine metal powders, said apparatus comprising a vacuum tight chamber, means for evacuating the chamber, means within said chamber for holding a charge of the metal to be evaporated, means for evaporating said metal charge, means providing for external cooling of said vacuum chamber, means providing for the removal of said fine metal powders from the walls of said chamber after condensation of the metal vapors to form the fine metal powders and means providing for the collection and treatment of said fine metal powders.

2. The apparatus of claim 1 wherein the means providing for the removal of said fine metal powder from the walls of said chamber comprises a rotating brush means conforming in shape to the top and walls of said vacuum chamber.

3. The apparatus of claim 1 wherein the vacuum chamber is a vertical cylindrical chamber sufficiently large to prevent direct impingement of the metal vapors on the walls of said chamber and preventing scale formation.

4. The apparatus of claim 1 wherein the means providing for the collection of said fine metal powder comprises a removal chamber connected to said vacuum chamber by a vacuum tight valve.

5. Apparatus for the production of fine metal powders, said apparatus comprising a vacuum tight chamber, means for evacuating the chamber, means within said chamber for holding a charge of the metal to be evaporated, means 5 for evaporating said metal charge, means providing for the removal of said fine metal powders from the Walls of said chamber after condensation of the metal vapors to form the fine metal powders, means providing for the collection of said fine metal powders, and means for removing a batch of collected powder from the vacuum chamber without introducing any contaminating atmosphere into the chamber.

6. The apparatus of claim 5 wherein said removed means comprises a vacuum tight valve, a second chamber which can be isolated from said vacuum chamber and from the atmosphere, and means for introducing a predetermined atmosphere into said second chamber.

195,709 Hallock Oct. 2, 1877 Newell Jan. 27, Marx Mar. 2, Case May 18, Bakken et a1. Aug. 3, Bakken Aug. 3, Grine June 10, Kemmer Aug. 25, Livingston Oct. 29, Hansgirg Mar. 16, Bancroft Feb. 5, Flosdorf et al Aug. 26, Moore Ian. 20, Camescasse et a1 Feb. 12, lshizuka Mar. 17, 

1. APPARATUS FOR THE PRODUCTION OF FINE METAL POWDERS, SAID APPARATUS COMPRISING A VACUM TIGHT CHAMBER, MEANS FOR EVACUATINNG THE CHAMBER, MEANS WITHIN SAID CHAMBER FOR HOLDING A CHARGE OF THE METAL TO BE EVAPORATED, MEANS FOR EVAPORATING SAID METAL CHARGE, MEANS PROVIDING FOR EXTERNAL COOLING OF SAID VACUUM CHAMBER, MEANS PRO- 